This invention relates to a photosensitive polyamide printing plate superior in contact to a negative or positive image-bearing film.
In photosensitive printing plates in may cases, a photosensitive polymer layer and an image-bearing film (negative or positive) are brought into close contact with each other, and in this state there is performed an exposure with an active light ray. In many cases, however, the surface of the photosensitive polymer layer has a stickiness, so that its uniform contact to the image-bearing film is impeded and it becomes impossible to make uniform printing of images. Particularly, a photosensitive polyamide layer comprising a polyamide as the main component, a photopolymerizable monomer, a plasticizer, a photosensitizer and a heat stabilizer, often has a sticky surface. This is ascribable to the circumstances that polyamides generally have a high equilibrium moisture content so become sticky easily, that there is contained a large amount of a photopolymerizable monomer which is in many cases liquid for attaining a high image reproducibility and that a liquid plasticizer is often incorporated to impart flexibility.
Various methods have been adopted to solve the abovementioned problem. According to the most generally adopted method, a very small amount of a powder is applied uniformly onto the surface of a photosensitive polymer layer whereby it is intended to remove the stickiness of the surface of the same layer and at the same time improve the air escape when an image-bearing film is brought into close contact in vacuum. But this method involves a drawback such that a fairly high degree of skill is required to apply a powder onto the surface of the photosensitive layer uniformly and in a very small amount. If the powder is applied nonuniformly, the uniform contact of the image-bearing film is impeded. And if the powder is applied in an excess amount, there is created a distance between the image-bearing film and the photosensitive layer, so that an active light ray will be scattered and a sharp image is no longer obtainable. Particularly in the case of a very sticky photosensitive printing plate such as a photosensitive polyamide printing plate, it is extremely difficult to a powder uniformly and in a very small amount.
According to another method, a matted film is used as the image-bearing film. In this case, the matted film is more expensive and is more difficult to manufacture than ordinary films. Moreover, in case the stickiness of a photosensitive layer is very high, a mere matting of the image-bearing film is in many cases unsatisfactory in point of the effect of improvement of contact. Additionally, if an image-bearing film is matted to an excessive degree in an effort to improve its contact, an active light ray will be scattered on the matted film surface and the resulting bad influence will become no longer negligible.
As a method of improving the contact between an image-bearing film and a photosensitive polymer layer there has been proposed a method wherein the mat surface of a matted film is brought into pressure contact with the photosensitive polymer layer thereby matting the surface of the same layer (see British Pat. No. 1,464,941). This proposed method is superior in that the operation is not difficult unlike the operation of powder application and in that ordinary films are employable. However, in case the surface of a photosensitive polyamide layer has a strong stickiness, a mere matting of the surface of the same layer is not effective enough to prevent such a stickiness. And if the matting degree is made excessively strong in an effort to improve the contact between a very sticky photosensitive layer and the image-bearing film, there comes into problem the bad influence based on the scattering of an active light ray on the surface of the photosensitive layer. Moreover, in case the surface of a photosensitive layer is remarkably sticky, the separation of the matted film itself becomes difficult and consequently it becomes necessary to apply a releasing agent such as silicone onto the mat surface (see U.S. Pat. No. 3,891,443).